Conventionally, ultrasonography is known as a method of nondestructively testing a structural member of an aircraft or the like. Furthermore, a technique using an FBG (fiber Bragg grating) sensor as a sensor for ultrasonic testing is also known. Using an FBG sensor as a sensor for receiving an ultrasonic wave can improve detection accuracy of ultrasonic waves compared to a case where an ultrasonic transducer is used as the sensor.
Thus, a technique of forming a groove on a metallic substrate by pressing a bar-shaped member against the metallic substrate so that an optical fiber sensor can be inserted into the metal groove has been proposed (for example, refer to International Publication No. WO 2008/123285 A1). With this technique, an optical fiber sensor can be embedded between metals joined by hot pressing.
On the other hand, a composite material, such as GFRP (glass fiber reinforced plastics) or CFRP (carbon fiber reinforced plastics), which is lightweight and has high strengths, has attracted attention mainly as a material of an aircraft structural object or the like. A composite material is manufactured by laminating and curing semi-cured sheet-like prepregs, each of which made of fibers, such as carbon fibers or glass fibers, impregnated with resin.
Thus, a technique of embedding an optical fiber, as a strain sensor, between sheet-like prepregs, which are a material before curing of a composite material, and then, curing the prepregs between which the optical fiber is embedded has been proposed (for example, refer to Japanese Patent Application Publication JP 2012-020483 A and Japanese Patent Application Publication JP H11-142262 A). Specifically, a composite material, in which an optical fiber has been embedded, can be manufactured by inserting the optical fiber between prepreg sheets during laminating the prepregs, precedential to curing the prepreg sheets.
However, the thickness of a typical optical fiber sensor is 100 μm order, thereby rigidity is low. Accordingly, when an optical fiber is inserted between prepreg sheets before curing, there is a problem that the optical fiber moves from an appropriate position by shrinkage of a composite material due to the curing.
Thus, an object of the present invention is to allow embedding an optical fiber sensor at an appropriate position, without deteriorating strength of a composite material.